HCV is an RNA virus belonging to the Hepacivirus genus in the Flaviviridae family. The enveloped HCV virion contains a positive stranded RNA genome encoding all known virus-specific proteins in a single, uninterrupted, open reading frame. The open reading frame comprises approximately 9500 nucleotides and encodes a single large polyprotein of about 3000 amino acids. The polyprotein comprises a core protein, envelope proteins E1 and E2, a membrane bound protein p7, and the non-structural proteins NS2, NS3, NS4A, NS4B, NS5A and NS5B.
The nonstructural protein NS5A is a membrane-associated phosphoprotein present in basally phosphorylated and hyperphosphorylated forms. It is a critical component of HCV replication and is believed to exert multiple functions at various stages of the viral life cycle. A full-length NS5A protein comprises three domains—namely, Domain I, Domain II, and Domain III. Domain I (residues 1 to 213) contains a zinc-binding motif and an amphipathic N-terminal helix which can promote membrane association. Domain II (residues 250 to 342) has regulatory functions, such as interactions with protein kinase PKR and PI3K, as well as NS5B, and also contains the interferon sensitivity-determining region. Domain III (residues 356 to 447) plays a role in infectious virion assembly, and can be modulated by phosphorylation within the domain. NS5A has been identified as a promising therapeutic target for treating HCV.